A ‘pinhole’ in enamel on a wire is a spot of insufficient electric strength to pass certain tests. The B.S. method of counting pinholes requires the wire to be passed through a mercury bath. This is considered destructive because it contaminates the enamel surface. A non-destructive method is described in which the wire is run through an open tube held at a sufficiently high voltage for corona to occur on the passage of a pinhole. It may be applied on enamelling plant and winding machines.Corona onset conditions are discussed and a formula is derived for the minimum electric stress to be applied to the bare wire surface. Peek's empirical law for the onset of visible corona between coaxial electrodes is recommended for calculating the maximum permissible surface stress.A suitable high-voltage supply unit and pulse-detecting and -counting circuits are suggested. Attention is drawn to the data-handling problem arising from the accumulation of a large amount of test information which must be reduced to an accept/reject decision. The solution adopted in a multi-channel instrument is based on the use of magnetic-core sealers and paper-tape distribution recorders.